F220.7. Project Summary/Abstract Women experience more voice disorders than men. This sexual dimorphism may, in part, be due to differences in sex hormones. However, the precise effects of hormones, such as estrogen, on the voice have been debated for decades. There is some evidence that changes in hormonal levels directly affect the vocal folds. For example, loss of estrogen is thought to be responsible for thickening of the outer vibratory layers of the vocal fold and, consequently, decrease the speaking fundamental frequency in post-menopausal women. The effects of menopause and other hormonal fluctuations on laryngeal muscles, however, is unknown. The proposed research will use a rat model to evaluate the effects of estrogen depletion on vocal function, by examining ultrasonic vocalizations (USVs) acoustics, as well as on underlying laryngeal neuromuscular mechanisms, by examining morphological and histochemical properties of the thyroarytenoid (TA) muscle, the primary muscle of the vocal fold. Estrogens affect both the structure and function of the female rat larynx. Surgically-induced menopause in female rats increases vocal fold inflammation, edema, and vasodilation, and reduces the number of vocalizations produced during mating. Estrogen may also mediate increased motor endplate cluster fragments in the neuromuscular junction (NMJ) of the TA muscles in female rats. Furthermore, menopause decreases power and endurance of fast-twitch hindlimb muscles by reducing parvalbumin levels, a protein necessary for calcium reuptake during muscle contraction. Therefore, vocal changes that occur during hormonal shifts are likely mediated by estrogen-dependent adaptations of both the superficial vibratory layers of the vocal folds and the intrinsic laryngeal muscles. We hypothesize that 1) lower estrogen levels will correspond with fewer USVs produced, increased USV fundamental frequency, reduced frequency bandwidth, and reduced frequency modulation rate, and 2) menopause will reduce motor endplate cluster fragments within the NMJs and reduce parvalbumin levels in the TA muscles. We will test these hypotheses in a rat model by first comparing USV acoustics across the estrous cycle in young adult female rats. We will then compare USV acoustics, NMJ morphology, and parvalbumin levels of the TA muscles between ovariectomized (estrogen-depleted) and control female rats. The proposed research has two specific aims: 1) to determine how the naturally-occurring estrous cycle affects USV production in young rats, and 2) to determine the effect of surgically-induced menopause on acoustic parameters of USVs and morphological and histochemical properties of the TA muscles. The proposed research is significant because it will provide foundational evidence to help explain clinical observations of low estrogen levels adversely impacting the female voice.